I don't think it has anything to do with westernisation. It's the same reason as women of all races in every country like minorities for casual relationships, including Australia. There is less chance of marriage, so they can enjoy the relationship without long-term implications. It's the same difference between dating some woman you picked up in a bar, and dating the girl-next-door. (Of course, then one or both people in the relationship fall in love, causing problems). Maybe not the case for every Japanese woman, but most just want the experience of making it with a foreigner, and I have no complaints about that!